1. Field of the Invention
The present invention relates to a composite electrical circuit protection device for protecting against overvoltage/overcurrent transient conditions and including a planar metal oxide varistor (“MOV”) overvoltage element that is critically coupled thermally to a planar polymeric positive temperature coefficient (“PPTC”) overcurrent element such that heat generated in the MOV element is effectively transferred to trip the PPTC element before the MOV element is irreversibly damaged, and without irreversible damage being caused to the PPTC element.
2. Introduction to the Invention
It is known in the art to provide composite circuit protection devices including overvoltage protection elements and overcurrent protection elements in thermally coupled relationships. Where thermal coupling between such elements has been provided, the design approach has been to maximize the transfer of heat from a heat-generating element to a heat-triggered element. One known example of an overvoltage protection element is a metal oxide varistor or “MOV”. One known example of an overcurrent protection device is a thermistor. For example, a composite device employing series-connected thermistors and parallel or shunt connected MOVs is described in U.S. Pat. Nos. 5,379,176 and 5,379,022, wherein thermistors and varistors, formed as solid cylindrical slugs from bulk material (e.g., barium titanate for the thermistor), are joined end-to-end by sheet metal spacers to form a composite circuit protection device “for optimizing heat transfer” in order to protect electronic measurement devices such as a digital multimeter from out-of-range overvoltage impulse events and out-of-range overcurrent conditions.
Another example of a composite thermistor-varistor protection device is set forth in U.S. Pat. No. 6,282,074. Therein, FIGS. 3 and 4 illustrate a fuse device comprising a MOV cylindrical element surrounding and directly contacting an interior cylindrical PPTC element.
MOVs are voltage-dependent, non-linear electrical elements typically composed primarily of zinc oxide with small trace amounts of other metals and oxides. The mixed materials comprising the MOV are formed by application of intense pressure and temperature in a sintering operation and thereby shaped into a final physical form such as a thin disk having a complex zinc oxide micrograin structure. Major surfaces of the MOV are provided with conductive metal (e.g., copper or silver-glass) formations or depositions to enable terminal leads or other connections to be made thereto. Desirably, MOVs have electrical I-V characteristics which resemble avalanche breakdown characteristics of back-to-back-connected zener diodes. Since each MOV in effect comprises a multiplicity of semiconductor junctions at the zinc oxide grain boundaries, the MOV acts very rapidly in response to an overvoltage condition, generating potentially a considerable amount of heat across substantially the entire disk surface while clamping the voltage to a nominal level. Thus, it would be desirable to transfer this distributed heat in an effective manner to a slower-acting overcurrent protection element, most preferably in the form of a polymeric positive temperature coefficient (“PPTC”) resistor element thereby to accelerate trip of the PPTC resistor device to its very high resistance state.
Stand-alone polymer PTC devices are well known. Particularly useful devices contain PTC elements composed of a PTC conductive polymer, i.e. a composition comprising an organic polymer and, dispersed or otherwise distributed therein, a particulate conductive filler, e.g. carbon black, or a metal or a conductive metal compound. Such devices are referred to herein as polymer PTC, or PPTC resistors, PPTC devices and/or PPTC elements. Suitable conductive polymer compositions and structural components, and methods for producing the same, are disclosed for example in U.S. Pat. Nos. 4,237,441 (van Konynenburg et al.), 4,545,926 (Fouts et al.), 4,724,417 (Au et al.), 4,774,024 (Deep et al.), 4,935,156 (van Konynenburg et al.), 5,049,850 (Evans et al.), 5,250,228 (Baigrie et al.), 5,378,407 (Chandler et al.), 5,451,919 (Chu et al.), 5,747,147 (Wartenberg et al.) and 6,130,597 (Toth et al.), the disclosures of which are hereby incorporated herein by reference.
As used herein, the term “PTC” is used to mean a composition of matter which has an R14 value of at least 2.5 and/or an R100 value of at least 10, and it is preferred that the composition should have an R30 value of at least 6, where R14 is the ratio of the resistivities at the end and the beginning of a 14° C. range, R100 is the ratio of the resistivities at the end and the beginning of a 100° C. range, and R30 is the ratio of the resistivities at the end and the beginning of a 30° C. range. Generally the compositions used in devices of the present invention show increases in resistivity that are much greater than those minimum values.
Polymeric PTC resistive devices can be used in a number of different ways, and are particularly useful in circuit protection applications, in which they function as remotely resettable devices to help protect electrical components from damage caused by excessive currents and/or temperatures. Components which can be protected in this way include motors, batteries, battery chargers, loudspeakers, wiring harnesses in automobiles, telecommunications equipment and circuits, and other electrical and electronic components, circuits and devices. The use of PPTC resistive elements, components and devices in this way has grown rapidly over recent years, and continues to increase.
It is known to provide PPTC resistor devices or elements in protective electrical connection and thermal contact with electronic components such as zener diodes, metal oxide semiconductor field effect transistors (MOSFETs), and more complex integrated circuits forming voltage/current regulators, as exemplified by the teachings and disclosures set forth in commonly assigned U.S. Pat. No. 6,518,731 (Thomas et al.), the disclosure of which is incorporated herein by reference. Also, see for example U.S. Pat. No. 3,708,720 (Whitney et al.) and U.S. Pat. No. 6,700,766 (Sato). Also note commonly assigned U.S. Pat. No. 4,780,598 (Fahey et al.) which describes PPTC elements that are thermally coupled by thermally conductive electrical insulator material to other circuit elements such as a voltage dependent resistor.
When sufficient current passes through a PPTC device, it reaches a critical or trip value at which a very large proportion of the heating (and voltage drop) nearly always takes place over a very small proportion of the volume of the device. This small proportion is referred to herein as the “hot line” or “hot zone”, see, e.g. U.S. Pat. No. 4,317,027 (Middleman et al.). It is generally understood that increasing the thickness of the PPTC layer will increase a protection device's ability to withstand higher voltages, but we have discovered that merely scaling the thickness of the PPTC layer using existing device geometries has not led to satisfactory high voltage circuit protection devices. Thus in order to realize an improved circuit protection device, it would be desirable to combine the overcurrent protection properties of the PPTC resistor element with the overvoltage protection properties of the MOV in an effective way that synergistically realizes full benefit of both protection elements in the single composite device.
Other PTC materials are also known, e.g. doped ceramics such as barium titanate, but are not as generally useful as PTC conductive polymer material in power protection applications, in particular because ceramics have higher non-operating, quiescent resistivities and also have Curie transition temperature levels that are higher than the transition temperatures associated with the trip to a high resistance state of a PPTC resistor.
In the telecommunications field, tip and ring wires of a communications pair may inadvertently induce or come into direct contact with a source of high voltage potential, such as a lightning strike or AC power induction or contact. Telecom protection devices must be capable of withstanding the high voltages and resultant high currents encountered in such events. Heretofore, leaded-style PPTC devices have been employed in high voltage electrical applications, particularly in the telecommunications field. Traditional leaded-style devices route current from the circuit board up through the leads to the metal foil electrodes. The leads serve as the terminals and the interconnection to the PPTC device's metal foil electrodes. Since the prior leaded PPTC devices are symmetrical, electrical conduction occurs in a direction through the PTC composite material that is normal or perpendicular to the oppositely facing metal foil electrodes. Thus, a thermal hot zone (and zone of maximum potential difference) is nominally formed as a thin planar region generally equidistant from, and parallel to, the metal foils of the PPTC resistor.
While the teachings of U.S. Pat. No. 6,282,074 noted above illustrate a PPTC cylindrical layer in direct contact with a MOV cylindrical layer within a bolt-shaped fuse structure, we have discovered that satisfactory results have not been obtained by optimizing or maximizing thermal transfer from a MOV element to a PPTC element, such as by placing a planar PPTC laminar device in direct contact with a facing planar surface of a MOV device, without a high likelihood of composite device failure. We attribute this likelihood of failure directly to the fact that when a major foil electrode of the PPTC element is positioned in direct contact with a major face of the MOV, heat generated within the MOV causes the PPTC resistor's hot zone to move closer to the major foil electrode, leading directly to PPTC element voltage breakdown and consequent failure.
Conversely, if the thermal coupling between the PPTC element and the MOV element is poor or essentially non-existent, the MOV element can fail due to excessive current flow caused by the overvoltage event and the failure of the PPTC element to heat up and trip in sufficient time to protect the MOV from irreversible failure.